1. Field of Invention
The present invention relates to a bistable liquid crystal device having a memory capability which uses a nematic liquid crystal, a driving method for driving the crystal device, and an electronic apparatus using the liquid crystal device.
2. Description of Related Art
A bistable liquid crystal display using a nematic liquid crystal has already been disclosed in Japanese Examined Patent Publication No. 1-51818. An initial alignment condition, two stable states, and a method for implementing the states are described therein.
In Japanese Examined Patent Publication No. 1-51818, however, only the operations or phenomenon of the two stable states are described, and there is no descriptions on means for practically using the states for a display apparatus. In addition, there is no descriptions on a matrix display, which has now the highest practical capability as a display apparatus and has a high display capability in the publication. A driving method for driving the liquid crystal device is not disclosed either.
The inventors have proposed in Japanese Unexamined Patent Publication No. 6-230751 a method for avoiding the foregoing drawbacks, in which backflow generated in a liquid crystal cell is controlled. In this method, a period in which the Freedericksz transition is generated by applying a high voltage for about one millisecond and immediately after that, a 0-degree uniform state is formed by the use of a constant voltage pulse which is equal to or higher than a threshold voltage with the same or reverse polarity as or to that of the foregoing pulse. Alternatively, in the same way, a period is provided immediately after the Freedericksz transition voltage, in which pulses equal to or lower than the threshold voltage are generated to implement a 360-degree twist state. In this method, the time required for writing per line in a matrix display is 400 xcexcsec. To write for 400 lines or more, a total time of 160 msec (6.25 Hz) or more is required and this causes a flicker in a display. A practical problem remains in this method.
Therefore, the inventors filed Japanese Unexamined Patent Publication No. 7-175041 to improve the writing time. As shown in FIG. 2 or FIG. 4 in that publication, a delay period is provided after the reset pulse which causes the Freedericksz transition, and then an ON or OFF selection signal is applied. With this method, the writing time can be reduced, for example, to 50 xcexcsec, which is about several times faster than before.
To make driving of bistable liquid crystal practical, some points are to be improved in addition to the writing time described above.
One of the issues is to implement to display all display patterns which may be displayed on a matrix display screen.
In the method for improving the writing time described above, for example, a scanning voltage signal supplied to the scanning signal line corresponding to a horizontal line has a reset period, a selection period, a non-selection period, and in addition, a delay period disposed between the reset period and the selection period. In this delay period, a voltage depending on the data potential of a pixel in a vertical line (data signal line) is applied to the liquid crystal in the same way as in the non-selection period.
The display patterns which may be displayed described above include an all black or white display pattern in one vertical line, a display pattern in which only one white or black dot is disposed in one vertical line, and a stripe display pattern in which white and black alternate in every dot in one vertical line. In the delay period, the voltage depending on each of these display patterns is applied to the liquid crystal.
It is found from experiments performed by the inventors, which will be described later as comparative examples in detail, that a selection voltage which allows the three display patterns described above to be displayed cannot be specified when the delay period is provided in a scanning signal used in the conventional driving method to drive a bistable liquid crystal. It is supposed that this is caused by a DC voltage application due to an unbalanced polarity of a voltage applied to the liquid crystal in the delay period.
Another issue is related to the power consumption of the bistable liquid crystal which is being driven. To drive the bistable liquid crystal, the preceding writing state needs to be reset in advance before the selection period. In the reset period, it is necessary to apply a reset voltage which is higher than that for other liquid crystal, for example, 25 V. This high reset voltage increases the power consumption of the bistable liquid crystal which is being driven. Therefore, if the power consumption increases due to the improvement of a driving method of the bistable liquid crystal, the driving method cannot be made practical.
Accordingly, an object of the present invention is to provide a liquid crystal device, a driving method therefor, and an electronic apparatus using the liquid crystal device, in which various types of display patterns can be displayed with a predetermined driving voltage margin being maintained and power consumption being prevented from increasing.
According to the present invention, a driving method for a liquid crystal device which includes a first substrate having a plurality of scanning signal lines, a second substrate having a plurality of data signal lines, and a liquid crystal disposed between the first and second substrates, in which a liquid crystal molecule has a predetermined twist angle at an initial state and there exist two metastable states different from the initial state as relaxation states generated after a voltage for bringing about a Freedericksz transition is applied, the method including:
supplying a scanning signal having a reset period, a delay period, at least one selection period, and a non-selection period in one vertical scanning period to each of the scanning signal lines;
supplying a data signal having the data potential corresponding to a display pattern to each of the data signal lines every time in the at least one selection period;
applying a voltage difference between the data signal and the scanning signal to the liquid crystal which is set to a reset potential at the reset period, set to a selection potential at the at least one selection period, and set to a non-selection potential at the delay period and at the non-selection period;
applying a reset voltage higher than or equal to a threshold value to the liquid crystal for bringing about the Freedericksz transition in the reset period according to the reset potential of the scanning signal and the data potential of the data signal;
applying a delay voltage to the liquid crystal in the delay period after the reset period according to the non-selection potential of the scanning signal and the data potential of the data signal;
applying a selection voltage to the liquid crystal for selecting one of the two metastable states in the at least one selection period after the delay period according to the selection potential of the scanning signal and the data potential of the data signal;
applying a non-selection voltage to the liquid crystal at the non-selection period following the at least one selection period according to the non-selection potential of the scanning signal and the data potential of the data signal;
setting the length of the at least one selection period to one horizontal scanning period (1H), and respectively setting the selection potential of the scanning signal and the data potential of the data signal corresponding to each selection period to positive and negative potential levels reversed in the positive and negative sides relative to the reference potential at an interval of 1H/m (m is an integer equal to or more than 2) so that a voltage of one polarity is not applied to the liquid crystal exceeding a 1H period irrespective of the display pattern in the delay period, the selection period, and the non-selection period; and
reversing the reset voltage in polarity in the positive and negative sides relative to the reference potential at an interval of a period longer than one horizontal scanning period (1H).
A device according to the present invention is defined as a liquid crystal device which implements the above method.
The present invention implements all display patterns which include, for example, an all black or white display pattern in one vertical line, a display pattern in which only one white or black dot is disposed in one vertical line, and a stripe display pattern in which white dots and black dots alternate in one vertical line. It was found from experiments of the inventors that if the voltage applied to the liquid crystal during the delay period continues to be applied with one polarity, an adverse effect appears which impedes display selection during the selection period following the delay period. Therefore, according to the present invention, a voltage with one polarity is not applied to the liquid crystal for more than a period of 1H during the delay period immediately before the selection period, irrespective of the display pattern which determines the display state of the liquid crystal. As a result, all these display patterns are allowed to be displayed.
To this end, the selection potential of a scanning signal and the data potential of a data signal are set to positive and negative potential levels alternately changed between the positive and negative sides at an interval of 1H/m (m is an integer equal to or more than 2) relative to the reference potential. In addition, the reset voltage applied to the liquid crystal during the reset period is alternately changed between the positive and negative sides at an interval of a period longer than one horizontal scanning period (1H). Since an increase of the number of times the polarity of the reset voltage alternates, which is relatively high, is prevented in this way, the total amount of the current which flows when the polarity of the reset voltage alternates is reversed is reduced and an increase of power consumption is also prevented.
It is preferred that the polarity of the reset voltage be changed at an interval of the vertical scanning period, or at a cycle of 2H or more. In this case, since the number of times the polarity of the reset voltage alternates, which is high, is reduced, power consumption is reduced.
It is preferred that the reset period of the scanning signal be divided into a plurality of periods, including at least a first period to a third period, be set to the positive or negative potential level having different polarities from each other relative to the reference potential in the first and third periods, and be set to the reference potential in the second period.
In this case, a voltage to be applied between adjacent scanning electrodes can be reduced. Even if the distance between adjacent scanning electrodes becomes short, it is unnecessary to have a large insulation voltage between the electrodes.
The present invention can also be applied to an MLS (multi-line selection) driving method. In this case, a scanning signal has a plurality of selection periods in one vertical scanning period. In the MLS driving method, the selection voltage is applied at the same time to the liquid crystal corresponding to a plurality of different scanning electrodes in each selection period. Each data potential of a data signal corresponding to each selection period of a scanning signal is set to a positive or negative potential level alternately changed between the positive and negative sides relative to the reference potential at an interval of 1H/m.
The data potential of a data signal used in the MLS driving method is determined by a combination of each of the display states of the simultaneously selected lines and set to the same potential as the reference potential in the data potential. It was found that, with the synergy of this condition and the condition in which the data potential is reversed at an interval of 1H/m, a wide driving voltage margin can be obtained. Since a one-polarity voltage is not continuously applied to the liquid crystal during the delay period irrespective of the display pattern, various types of display patterns can be easily displayed.
It is preferred that a scanning signal used in the MLS driving method has an interval period as the reference potential, between two selection periods provided in one vertical scanning period. With this setting, it can be set that a one-polarity voltage is not applied to the liquid crystal for more than a period of 1H irrespective of the display pattern.
It is preferred that the delay period be set to ranges from 210 xcexcsec to 700 xcexcsec. It was found that the saturation voltage Vsat and the threshold voltage Vth of a liquid crystal change according to the length of the delay period, and the voltage difference |Vsatxe2x88x92Vth| therebetween also changes. To generate the liquid crystal arrangement corresponding to a display ON state, an ON voltage applied to the liquid crystal needs to be higher than Vsat. To generate the liquid crystal arrangement corresponding to a display OFF state, an OFF voltage applied to the liquid crystal needs to be lower than Vth. It was found that the voltage difference |Vsatxe2x88x92Vth| needs to be small and the delay period needs to be set to that described above in order to satisfy these conditions. Therefore, the arrangement of the liquid crystal corresponding to the ON/OFF display state can be controlled by setting the length of the delay period as described above.